Biology ETDs

Publication Date

Spring 4-15-2019


Ponderosa pine forests evolved with low-severity surface fires to maintain forest structure and process. However, increasingly common high-severity fires often result in complete overstory canopy loss and a shift from forest to shrub- and grass-dominated vegetation that may persist as an alternate stable state due to limited rates of ponderosa seedling regeneration in severely burned areas. Though overstory tree canopy is an important driver of understory plant response, little is known about how the absence of tree canopy following severe fires interacts with changes in year-to-year climate to affect the physiological function of ponderosa pine seedlings. This field study tested whether canopy presence or absence after fire interacts with drought to affect the photosynthetic efficiency, carbon-water balance, and soil nitrogen of ponderosa pine seedlings across three growth years. Photosynthetic efficiency was measured with chlorophyll fluorescence, carbon-water balance was measured with carbon isotopes and stomata density, and soil nitrogen cycling was measured with nitrogen isotopes. Chlorophyll fluorescence results showed no significant difference in photosynthetic efficiency by canopy condition or growth year, though measurements throughout the season are needed to test this question further. Stomata density and carbon isotope results showed a disproportionate effect of overstory canopy absence on the carbon-water balance of seedling foliage in a drought year. Despite increased stomata density, seedlings without overstory canopy had significantly higher carbon isotope ratios. This was most likely due to decreased stomatal conductance as a result of water limitations during the drought. Nitrogen isotopes were also significantly higher for seedlings without overstory canopy, indicating increased rates of soil nitrification. Together these results indicate that canopy loss from high-severity fire has a physiological impact on developing tree seedlings, resulting in disproportionately greater water-stress in a drought year. This finding adds a new perspective on the importance of low-severity fire as a management strategy to increase ponderosa pine forest resiliency; by maintaining overstory canopy, seedlings may be buffered from the physiological stress associated with droughts, which are projected to become more common in the southwest as climate changes.




southwest, Jemez, chlorophyll fluorescence, carbon isotopes, nitrogen

Document Type


Degree Name


Level of Degree


Department Name

UNM Biology Department

First Committee Member (Chair)

David Hanson

Second Committee Member

Matthew Hurteau

Third Committee Member

Marcy Litvak

Included in

Biology Commons